Add support for WG21 N3599 (literal operator template for strings) as a GNU
extension. The GCC folks have decided to support this even though the standard
committee have not yet approved this feature.

Patch by Hristo Venev!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192128 91177308-0d34-0410-b5e6-96231b3b80d8